Road surface dressing is a relatively inexpensive technique for sealing and retexturing a worn road surface in order to improve its skid resistance. The procedure involves spraying a thin film of binder, for example bitumen, tar bitumen, bitumen emulsion or foamed bitumen, onto the old road surface, immediately spreading stone chippings or other aggregate onto the bituminous film and rolling the chippings into the film. The chippings may be used in an uncoated form (known as "dry") or they may have been previously coated with bitumen ("precoats").
A principal cause of surface dressing failure and chipping loss, especially in wet weather, is poor adhesion between the chippings and the sprayed binder film. This may be the result of any of a number of factors. For example, "dry" chippings are in practice often saturated with water or may absorb water after being spread on the road. Also, such chippings often have a dust film adhering to them, which may be present even after the stones have been washed, as a consequence of abrasion during transportation and handling. One method of overcoming this problem is to precoat the chippings with bitumen. However, precoated chippings may stick together, thereby blocking the chipping applicator, and the bitumen coating can strip from the chipping in wet conditions, allowing water to penetrate the stone.
All of these factors contribute to adhesion failure and in very cold or freezing conditions, water absorbed by the chippings can expand causing the chippings to lift or fracture.
Similar problems of poor adhesion between stone chippings and binder can occur also in hotmix (bitumen macadams, asphalt concretes, rolled asphalts, etc.) in which the stone chippings and the bitumen binder material are first mixed together at an elevated temperature and then spread onto the road surface and compacted, and in foamed bitumen applications which require the presence of large quantities of water.
Various proposals have been made over the years for improving the adhesion between the chippings and the binder, including the chemical treatment of the surface of the chippings and the inclusion in the bituminous binder of adhesion-promoting agents, usually nitrogen derivatives such as fatty amines. These latter materials, however, suffer from the disadvantage that they are rarely heat stable and their performance therefore diminishes rapidly during storage of hot bitumen containing them. The present invention, however, now seeks to provide a method for physically treating stone chippings and other aggregate materials by a simple and inexpensive procedure which increases the adhesion between the chippings and the binder.